This invention relates to a field emission cathode structure, a method for the production thereof, and a flat panel display device using the cathode.
In recent years, the advanced technology on the fabrication of Si semiconductors has been lending itself immensely to the development of field-emission type cathode structures and to the utilization of these cathodes in ultraspeed microwave devices, power devices, electron beam devices, flat panel display devices, etc. As a typical example of the cathode, what has been reported by C. A. Spindt et al. in Journal of Applied Physics, Vol. 47, No. 12, December 1976, pages 5248-5263 has been known to the art.
The field emission cathode structure disclosed therein, as illustrated in FIG. 9a, FIG. 9b, and FIG. 9c of the present specification, is produced by forming a SiO.sub.2 layer 2 as an insulating layer by the technique of deposition such as CVD on a Si single crystal substrate 1, further forming thereon a Mo layer 3 destined to serve as a gate electrode layer as by the electron beam vacuum deposition technique, boring a pinhole 5 approximately 1.5 .mu.m in diameter through the layers 2 and 3 by means of etching, then forming an Al layer 4 destined to serve as a separating layer by means of vacuum deposition (FIG. 9a), vacuum depositing thereon a metal such as Mo which is destined to form an emitter as by the technique of electron beam vacuum deposition while keeping the Si single crystal substrate 1 in rotation thereby giving rise to a conical pile of Mo inside the pinhole 5 by utilizing the phenomenon that the diameter of the pinhole 5 converges in proportion as the deposition of Mo proceeds (FIG. 9b), and finally finishing a conical emitter 7 by peeling the Al separating layer 4 and removing the Mo layer 6 (FIG. 9c).
The electronic device using a cathode structure such as, for example, the flat panel display device is constructed by causing a Si single crystal substrate 1 having a multiplicity of such cathodes superposed thereon to be opposed across a prescribed interval to a glass face plate 8 having a phosphor layer superposed thereon as illustrated in FIG. 10. In this diagram, A stands for a region for the formation of cathodes. This flat panel display device using field emission cathodes is different from the display device using a liquid crystal in being a luminescent type. It obviates the necessity for using a back light and consequently promises a saving in power consumption. Owing to these features, it has been attracting keen attention.
The conventional method for producing the field emission cathode system, the field emission cathode structure obtained by the method, and the electronic devices using such cathode structures, however, entail the following important problems.
Firstly, in the conventional method for rotary vacuum deposition described above, since the formation of the emitter 7 inside the pinhole 5 is attained by utilizing the phenomenon that the diameter of the pinhole 5 bored in the Mo layer 3 gradually converges, the height of the emitter and the shape of the tip of the emitter are liable to loss of consistency. The cathode structure which is obtained by this method, therefore, produces field emission with poor uniformity and lacks the sharpness of the tip of the emitter which is necessary for improving the efficiency of field emission and, as a result, entails such problems as decline of the efficiency of field emission and growth of power consumption. Further, the fact that the reproducibility of shape and the yield of production are both inferior gives rise to a problem of extremely high cost of production in the fabrication of a multiplicity of field emission cathode structures on one and the same substrate.
Secondly, since the SiO.sub.2 insulating layer is formed by the technique of CVD, the distance between the gate and the emitter on which the efficiency of field emission heavily hinges defies accurate control, and the magnitudes of field emission which the plurality of cathode structures severally generate lack uniformity. In the production of a flat panel display device, for example, the picture elements corresponding to the individual cathode structures are suffered to betray inconsistency of luminance. As respects the flat panel display device, owing to the slight loss of consistency in the distance between the gate and the emitter and in the shape of the tip of the emitter, it often happens that the ratio of the current of electrons between the gate and the emitter to the current of electrons between the anode and the emitter increases. There are times when the current of electrons between the gate and the emitter even reaches 60% of the total current of electricity. Thus, the problem arises that the efficiency of light emission of the picture elements (fluorescent elements) corresponding to the individual cathode structures is degraded and, at the same time, the picture elements suffer from serious inconsistency in luminance.
Thirdly, the size of the Si single crystal substrate imposes a limit on the regions to be used for the formation of field emission cathode structures or the number of such cathode structures to be formed and, at the same time, impairs the productivity of the cathode structures. This fact implies that the flat panel display device using a multiplicity of cathode structures is limited in size. Further, the flat panel display device by nature is fated to use the Si single crystal substrate as part of the housing of the device. As a vacuum container, the housing is conspicuously deficient in strength. Especially, when the image screen grows in size, the housing retains required strength only with increasing difficulty.
Fourthly, since the emitter is formed by depositing the Si single crystal substrate or conductive substrate concurrently serving as a cathode, the continuity between the emitter and the cathode is disrupted along their interface no matter whether the material for the emitter and that for the cathode are different or same. Thus, the disadvantage arises that the emitter will peel and incur loss of resistance and, consequently, generate heat possibly to the extent of deteriorating the emitter itself.
For the sake of eliminating the limit on size and enhancing the strength of the housing, an idea of superposing the Si single crystal substrate fast on a structural substrate such as a glass substrate may be conceived. The mere superposition results in an addition to the thickness of the cathode part and proves unfit for such electronic devices which are directed toward decreasing weight and thickness, for example. The substitution of a glass substrate for the Si single crystal substrate indeed eliminates the problems on size mentioned above. It, however, necessitates formation of a conductive layer on the glass substrate for the purpose of ensuring maintenance of conductivity to the emitter. Thus, the formation of the SiO.sub.2 insulating layer does not permit adoption of the technique of CVD but requires use of the technique of electron beam vacuum deposition or the technique of spattering. The SiO.sub.2 insulating layer which is obtained by such a technique, however, assumes a more porous texture and contains more pinholes than the layer obtained by the technique of CVD and suffers from aggravated inconsistency in the distance between the gate and emitter which governs the efficiency of field emission.
The flat panel display device using the conventional field emission cathode structures entails the following problems in addition to the problems pertaining to the process of manufacture of cathode structures mentioned above. When the field emission cathode structures are used, in spite of an increase in the voltage applied between the cathode and the anode to a level of about 100 V, the energy of the electron beam is small as compared with that of the ordinary color cathode ray tube (hereinafter referred as "C-CRT") and the fluorescent elements collect electric charge on their surfaces possibly to the extent of repelling the electron beam. Thus, the problem arises that the infiltration of electrons in the fluorescent elements occurs only to several nm from their surfaces and the fluorescent elements suffer from poor efficiency of emission. When the applied voltage is increased, the energy of the electron beam is augmented and the efficiency of emission is enhanced and, at the same time, the practicability of allowing use of fluorescent elements which avoid inducing saturation of luminance and excel in efficiency of emission is realized. The increased voltage, however, entails such problems as ionization of impurity gas and consequent sputtering of the surface of the cathode and breakdown of the insulation between the gate and the cathode. Since these circumstances compel the applied voltage lower than that normally used in the C-CRT, the efficiency of emission which is actually obtained is lower than that which is obtained inherently.
As a way of compensating, if only nominally, this decline of the luminous efficiency, the method which consists in coating the surface of the fluorescent face plate opposite the surface thereof intended for observation, namely the fluorescent rear surface, with aluminum thereby forming a so-called metal back and enabling the metal back to reflect the light radiated on the fluorescent rear surface side toward the observation surface side may be cited. This method has found extensive utility in the ordinary C-CRT devices. The use of the metal back, however, necessitates application of a high voltage even reaching the range from 6,000 V to 8,000 V for enabling the electron beam to pass through the Al layer and, consequently, entails the aforementioned problems of sputtering and breakdown of the cathode, and renders it extremely difficult to maintain mutual insulation of the anode and the cathode because the gap intervening between them is as narrow as to fall in the range of from several .mu.m to 1 mm.
As described above, the method for the production of the conventional field emission cathode structure poses various problems such as degradation and inconsistency of the efficiency of field emission and inferiority of the yield of production owing to the marked deficiency of the shape of the emitter in reproducibility and uniformity and, at the same time, entails such problems as restriction of the region for the formation of the field emission cathode structure by the size of the Si single crystal substrate and inevitable use of the Si single crystal substrate as part of the housing of the device.
With a view to improving the reproducibility and uniformity of the shape of emitter, Sokolich et al. (IEDM 90, pages 159-162) have developed a method for arranging uniform tips on a polysilicon substrate by forming in a &lt;100&gt; Si single crystal substrate by the etching technique a pyramidal hole having a sharp bottom tip, forming on the surface of the hole a thin oxide film as an etching barrier, subsequently depositing polysilicon in the Si mold wafer, and further removing the mold wafer by etching. The polysilicon tips thus produced are coated as with Mo, overcoated with a SiO.sub.2 film by the CVD technique, further coated with a layer of grid metal, and then finished as field emission cathodes.
This method of manufacture proposed by Sokolich et al., however, has the following problems. When the mold layer is used, the tips are discernibly improved in reproducibility and uniformity as compared with those obtained by the rotary vapor deposition method described above. The sharpness of the tips does not necessarily deserve to be called sufficient, because of the difficulty of coating emitter material on the sharp emitter tips, and the emitter material coated on polysilicon is sometimes peeled off. Further, since the SiO.sub.2 film is formed by the CVD technique, the gaps intervening between the gates and the emitters defy control and permit no easy decrease. Thus, it has been difficult for this film to be produced in high quality.
The flat panel display device which uses the conventional field emission cathode structure, even when the device is relieved of the problems which pertain to the emitter, still suffers structural problems such as inferior luminace of picture elements and excessive dispersion of luminance.